Wireless radio localisation is an area of technology that uses radio signals to determine the location of a device. The scope of this technology is very wide, varying from short range (a few meters) to very long ranges associated with the navigation of aircraft. In recent times the best known system is the US-originated Global Positioning System (GPS), which provides accuracy of the order of a few meters (or better) anywhere on the surface of the Earth, provided line-of-sight propagation exists to the associated satellites. However, indoor localisation or localisation in an urban environment is much less developed, mainly due to the difficult radio propagation conditions. One of the most common technologies is to use receiver signal strength. This can be used to estimate range, and hence perform localisation by multilateration; however, this gives very poor results in indoor environments. A better approach is to survey the signal strength at locations of interest and perform matching to determine location. This requires updating the survey whenever changes in the physical environment affect the radio propagation, and even with this updating, high accuracy is rarely achieved. Greater accuracy is achieved by measuring the time of arrival (TOA) (or in some cases the phase) of a radio signal. Such systems effectively estimate the time a “pulse” of radio energy is detected in the radio receiver. The accuracy of this determination depends upon many factors, the most important of which include the signal bandwidth, the signal-to-noise ratio (SNR), and the signal-to-interference (multipath) ratio. As a wideband system can result in the generation of a narrow pulse in the radio receiver, the accuracy of the system is essentially proportional to the signal bandwidth. In indoor and other environments with multipath signals, the received signal is a complex mixture of multiple scattered signals. As the scattered and reflected signals are delayed relative to the direct path, the accuracy of the measurement of the TOA reduces to the order of these delays. However, if the signal bandwidth is sufficient to resolve each of the signals, then the TOA measurement can be based on the arrival of the first significant signal without any corruption from the other scattered signals. Even where individual signals cannot be resolved, increasing the bandwidth still improves the accuracy of the TOA measurement. Accurate localisation in a multipath environment therefore favours wide bandwidth signals for TOA measurement. The problem with wide bandwidth is that it requires the use of complicated, power hungry, and relatively expensive radio transmitters and receivers.
One wideband technology for providing accurate TOA measurement is called Ultra-Wideband (UWB). UWB occupies a bandwidth from 3.1 GHz to 10.6 GHz; however, current government regulations severely limit the RF power radiated in this spectrum in order to avoid interference with other radio systems. The range of such systems is thus limited to about 10 meters. Such systems require a large number of base stations to cover a typical indoor area, so that installations can be expensive and logistically difficult. Such systems also require expensive radios to generate and receive the UWB signals.
In the case that the direct radio signal is not corrupted by unresolved reflections, peak detection is the optimum method for measurement of TOA. However, this lack of corruption is not usually the case, and other methods have been proposed for the measurement of TOA. These may be classed into two broad groups:    Methods based on analysis of the channel impulse response. An estimate of the channel impulse response can be determined by correlating the received signal against the transmitted signal or by taking the inverse Fourier transform of the channel frequency response. The TOA may be measured from the channel impulse response by a number of different schemes including: peak detection, leading edge detection based on thresholding, adaptive thresholding, and schemes based on an analysis of the slope of the leading edge.    Super-resolution schemes based on direct analysis of the channel frequency response. These methods attempt to find a small set of multipath delays and amplitudes that match the observed channel frequency response. Such schemes are extremely intensive computationally, typically requiring the calculation of eigenvalues and eigenvectors of large matrices. Furthermore, the improvement obtained over the time domain methods is marginal. The fundamental problem is that, in dense multipath environments, the channel frequency response contains insufficient information to reconstruct the set of arrival times. As a result, there are a number of possible solutions which match the observed data equally well, but give different estimates for the TOA.